Ultrahigh-molecular-weight polyethylene has better impact strength, abrasion resistance, chemical resistance and tensile strength than general high-molecular-weight polyethylenes, and its utility as engineering plastics has been on an increase. However, the ultrahigh-molecular-weight polyethyene has a much higher melt viscosity and lower flowability than the general-purpose polyethylenes. Hence, it has the defect of poor moldability and is extremely difficult to mold by extrusion or injection molding.
Accordingly, in most cases, articles from the ultrahigh-molecular-weight polyethylene are presently produced by compression molding. Some articles, such as rods, are produced only at very low speeds by extrusion molding.
Previously, as a method of improving the melt moldability of ultrahigh-molecular-weight polyethylene, the mixing of the ultrahigh-molecular-weight polyethylene with low-molecular-weight or high-molecular-weight polyethylene was proposed.
Japanese patent publication No. 27064/1971 discloses an abrasion-resistant polyethylene resin composition comprising polyethylene having an average molecular weight of at least 500,000 and 20 to 50% by weight of polyethylene having a density of at least 0.940 and an average molecular weight of 30,000 to 120,000.
Japanese Patent Publication No. 41,309/1983 discloses a polyethylene composition comprising 85 to 50 parts by weight of polyethylene having a viscosity average molecular weight of 500,000 to 1,500,000 and 15 to 50 parts by weight of particulate ultrahigh-molecular-weight polyethylene having a viscosity average molecular weight of at least 1,000,000 and a particle size smaller than 10 mesh.
Japanese Laid-Open Patent Publication No. 177,036/1982 discloses an ultrahigh-molecular-weight polyethylene composition having improved moldability comprising 100 parts by weight of ultrahigh-molecular-weight polyethylene having a molecular weight of at least 1,000,000 and 10 to 60 parts by weight of low-molecular-weight polyethylene having a molecular weight of 5,000 to 20,000. The specification of this Laid-Open Publication states that the moldability of the ultrahigh-molecular-weight polyethylene composition is such that in the production of a slab having a thickness of 50 mm by compression molding, it required a molding cycle of 200.degree. C..times.2 hours whereas the ultrahigh-molecular-weight polyethylene alone required a molding cycle of 200.degree. C..times.3 hours; and in ram extrusion molding method, the pipe extrusion speed likewise increased to 10 cm/min. from 5 cm/min.
The moldability of the above ultrahigh-molecular-weight polyethylene composition is improved as stated above, but should be further improved.
Japanese Laid-Open Patent Publication No. 126,446/1984 discloses an ultrahigh-molecular-weight polyethylene resin composition comprising 95 to 50 parts by wight of an ultrahigh-molecular-weight polyethylene resin and 5 to 50 parts by weight of a general-purpose polyolefin resin. The specification of this patent publication discloses a composition comprising a silane-modified polyethylene resin having a melt index of 2.5 or 5.0 g/l0 min. as an actual example of the general-purpose polyolefinic resin. The moldability of the composition is described in Table 1 of this specification, but it is not that the moldability is sufficiently good with regard to all compositions
The above polyethylene resin compositions are all prepared by mixing ultrahigh-molecular-weight polyethylene with polyethylene or a polyolefin having a lower molecular weight.
On the other hand, Japanese Laid-Open patent publication No. 94593/1979 (corresponding to U.S. Pat. No. 4,414,369) discloses a process for producing a polyolefin having a broad molecular weight distribution by polymerizing an olefin in the presence of a solvent and hydrogen using a Ziegler-type catalyst supported on a solid carrier and having enhanced activity; characterized in that
a plurality of reactors are used,
a major olefin monomer and at least one olefin comonomer are continuously fed into a first-stage reactor and copolymerized while a gaseous phase containing an inert gas is present in the upper portion of the reactor,
the polymerization reaction mixture in which high-molecular-weight polymer particles are dispersed in the solvent is transferred continuously by a difference in pressure into a second-stage reactor consisting of a vertical stirred vessel kept at a lower pressure than the first-stage reactor without substantially separating part of the components in the mixture and without using a forced transferring means,
in the second-stage stirred vessel, the polymerization is continuously carried out in the presence of the major olefin monomer and hydrogen while a gaseous phase is present in the upper part of the stirred vessel to form a polymer having a lower molecular weight than the polymer obtained in the first stage, and
the polymerization reaction mixture containing the resulting polymer particles dispersed in the solvent is continuously withdrawn from the second stirred vessel, and
the polymer is recovered from it.
Japanese patent publication No. 10,724/1984 (corresponding to U.S. Pat. No. 4,336,352) discloses a process for continuously producing polyethylenes having different molecular weights in a multiplicity of stages in three or more polymerization reactors connected in series. The purpose of this process is to produce polyethylene having excellent physical properties and moldability. The specification of this patent publication states in column 5, lines 28-30 that in order to achieve this purpose, the proportion of ultrahigh-molecular-weight polyethylene having a viscosity average molecular weight of at least 1,000,000 should be limited to 1 to 10%, preferably 1 to 7%, based on the total amount of the polyethylenes formed.
Japanese Laid-Open patent Publication No. 141409/1982 discloses a process for producing polyethylene which comprises polymerizing ethylene or copolymerizing ethylene with an alpha-olefin using a catalyst comprising the reaction product of a magnesium compound and a titanium halide and an organoaluminum compound; characterized in that
the following three polymerization steps are carried out in any desired order,
step (a): a step of forming an ethylene polymer or copolymer having an alpha-olefin content of not more than 10% by weight and an intrinsic viscosity [.eta.] of 0.3 to 1.5,
step (b): a step of forming an ethylene polymer or copolymer having an alpha-olefin content of not more than 30% by weight and an intrinsic viscosity [.eta.] of 1.5 to 7,
step (c): a step of forming an ethylene polymer or copolymer having an alpha-olefin content of not more than 30% by weight and an intrinsic viscosity [.eta.] of 7 to 40, and
the polymerization reactions are carried out while the amounts polymerized in the above steps, in terms of the weight ratio of step (a):step (b):step (c), are adjusted to 1:0.1-1.5:0.01-1.2.
Japanese Laid-Open patent Publication No. 8712/1983 discloses a process for producing an ethylenic copolymer which comprises copolymerizing ethylene and an alpha-olefin using a catalyst system obtained from (A) a solid catalyst component containing at least a magnesium atom, a halogen atom and a titanium atom and (B) an organoaluminum compound in two stages; characterized in that
(1) in at least one of said steps, 80 to 20 parts by weight of a copolymer having a high load melt index of 0.03 to 10 g/10 min. and a density of 0.890 g/cm.sup.3 to 0.905 g/cm.sup.3 is produced, PA1 (2) in the second step, 20 to 80 parts by weight of a copolymer having a melt index of 10 to 5000 g/10 min. and a density of 0.905 to 0.940 g/cm.sup.3 is produced, ps thereby to produce a copolymer having a melt index of 0.02 to 30 g/10 min. and a density of 0.890 to 0.935 g/cm.sup.3. PA1 (1) producing 80 to 20 parts by weight of a copolymer of ethylene with propylene and/or butene-1 having a high load melt index of 0.03 to 10 g/10 min. and a density of 0.890 to 0.935 g/cm.sup.2 in at least one of said stages, and PA1 (2) producing 20 to 80 parts by weight of a copolymer of an ethylene/alpha-olefin copolymer having a melt index of 10 to 5000 g/10 min. and a density of 0.890 to 0.940 g/cm.sup.3 in which the content of alpha-olefins having 5 to 12 carbon atoms in alpha-olefins having at most 12 carbon atoms as comonomers is at least 30 mole %, PA1 (a) in the case of a suspension polymerization, from 0.05 to 3.0 millimols per liter of dispersion medium, or in the case of a polymerization in the gaseous phase, from 0.05 to 0.3 millimol per 0.5 liter of reactor volume, of a chlorine-containing trivalent compound, and PA1 (b) from 0.1 to 3.0 millimols of aluminum per liter of dispersion medium or reactor volume, in the form of an aluminum trialkyl having the general formula AlR.sub.3 in which each R represents a hydrocarbon radical that contains from 4 to 40 carbon atoms, or in the form of the reaction product of an aluminium trialkyl or an aluminium alkyl hydride with a diolefin that contains from 4 to 20 carbon atoms.
The specification of this publication discloses that the high load melt index was measured at a temperature of 190.degree. C. under a load of 21.6 kg in accordance with JIS K-6760.
Japanese Laid-Open patent Publication No. 8713/1983 discloses a process for producing an ethylenic copolymer by polymerization in a multiplicity of stages using the same catalyst as described in the above-cited Japanese Laid-Open patent Publication No. 8713/1983, which comprises
thereby to form a copolymer having a melt index of 0.02 to 30 g/10 min. and a density of 0.890 to 0.936 g/cm.sup.3.
Japanese Laid-Open patent Publication No. 120605/1984 discloses a process for producing an ultrahigh-molecular-weight polyethylene-type resin having improved moldability which comprises polymerizing monomers using a Ziegler-type catalyst comprising an organometallic component and a solid catalyst component containing a transition metal component in at least two stages in which the monomer compositions and the hydrogen concentrations are different; characterized in that in one of said stages, propylene or a monomeric mixture comprising propylene as a main component, or butene-1 or a monomeric mixture comprising butene-1 as a main component is polymerized in the presence of hydrogen to produce 2 to 60%, based on the entire polymers, of a polypropylene or polybutene-1 component, and that in at least one of the other stages, ethylene or a monomeric mixture comprising ethylene as a main component is polymerized in the substantial absence of hydrogen to produce 98 to 40%, based on the entire polymers, of an ultrahigh-molecular-weight polyethylene component.
British Pat. No. 1,174,542 discloses a process for the preparation of a homo- or co-polymer of ethylene by a gaseous phase polymerization, or by a suspension polymerization in which the dispersion medium is in contact with a gaseous phase of ethylene or a mixture comprising ethylene and up to 10% by weight of an alphaolefin that contains from 3 to 15 carbon atoms, which process comprises preparing from 5% to 30% by weight of the total polymer in the presence of from 0% to 10% of hydrogen, calculated on the total volume of the gaseous phase, and preparing from 70% to 95% by weight of the total polymer in the presence of from 20% to 80% of hydrogen, calculated on the total volume of the gaseous phase, both stages of the polymerization being carried out at a temperature within the range of from 50.degree. C. to 120.degree. C. and a pressure of 10 atmospheres gauge, in the presence of a catalyst which is present in the first stage in an amount sufficient for both stages, said catalyst comprising
It is an object of this invention to provide a novel ultrahigh-molecular-weight polyethylene composition.
Another object of this invention is to provide a polyolefin composition having excellent melt moldability, particularly very good melt extrusion moldability, without impairing the excellent mechanical properties, such as high impact resistance or strength properties, inherent to ultrahigh-molecular-weight polyethylene.
Further objects and advantages of this invention will become apparent from the following description.